A 2-order MIMO Full-Duplex Antenna System Elpiniki Tsakalaki, Ehsan Foroozanfard, Elisabeth de Carvalho, Gert F. Pedersen † † The authors are with the Antennas, Propagation and Radio Networking Section, Aalborg University, 9220 Denmark Emails: Elpiniki.Tsakalaki.PhD@IEEE.org, efo@es.aau.dk, edc@es.aau.dk, gfp@es.aau.dk Abstract—The paper presents an antenna system with com- bined full-duplex and 2-order multiple-input-multiple-output (MIMO) functionalities, i.e., a system capable of spatially multi- plexing and spatially demultiplexing 2 datastreams in the same frequency and in the same time. By exploiting symmetries in the construct and the feed, simple corrective beamforming weights can be applied at the transmitter (Tx) ports in order to selectively cancel the Tx signal at the receiver (Rx) ports, thus guaranteeing a large amount of isolation necessary for full- duplex (i.e., simultaneous and in-band Tx and Rx) operation. On the other hand, the 2 MIMO ports (either at the Tx or at the Rx) are sufficiently decoupled thanks to polarization diversity. The proposed antenna system exhibits a remarkable level of full- duplex isolation over a wide bandwidth while maintaining low coupling between its MIMO ports and can serve as a concrete implementation of an antenna system equipped with both MIMO as well as full-duplex capabilities. Index Terms—Full-duplex systems, MIMO arrays, antenna isolation. I. I NTRODUCTION MIMO systems have been proposed for enhanced spectral efficiency by transmitting and receiving multiple datastreams over the same frequency and time utilizing multiple antennas at the Tx and / or the Rx terminal. While MIMO is becoming a well-established technology in existing and emerging wireless standards [1], [2], future development of wireless communi- cations networks will require additional radical improvements over the current radio transmission and network solutions to support the tremendous growth of data traffic [3]. To this end, full-duplex radio has attracted significant research attention by introducing a new communication paradigm where the same carrier frequency is used for concurrent transmission and reception at the same device. The approach holds the potential to significantly improve the radio network performance, e.g., by effectively doubling the single-link throughput through the utilization of the channel for simultaneous collision-free transmissions and receptions with full data capacity in both directions, by activating the concurrent source-relay and relay- destination links in multi-hop configurations and by facilitating wireless networking [4], [5]. Therefore, transceivers with com- bined full-duplex and MIMO technologies can significantly boost the capacity of wireless networks and help to meet the ever-increasing datarate demands. So far, the immense difference between the Tx power level (between 0−30 dBm, e.g., in WiFi systems) and the sensing radio sensitivity (around −100 dBm) has prohibited practical deployment of full-duplex systems [6]. Hence, the feasibility of full-duplexing depends solely on the ability of isolating the Tx path from the Rx path to a level which ensures that the Tx signal acting as a local self-interference does not affect the Rx sensitivity. Such immense level of required Tx-Rx isolation or full-duplex isolation (in the order of 100 dB) is much larger than the isolation level needed to decouple two Tx or two Rx antennas for MIMO capabilities (MIMO isolation). In order to provide Tx-Rx isolation, active filters have been proposed [5], [7]-[10]. Such digital or analogue filters utilize the knowledge of the local Tx signal to produce a signal that is added at the Rx signal to cancel the self-interference. The addition happens in the Rx baseband after the analogue-to- digital (ADC) conversion (baseband cleaning) [7], [8], or in the Rx radio-frequency (RF) stage (power cancellation) [5], [9], [10], respectively. To generate the cancellation signal, the Tx signal is properly digitally or RF modulated in both amplitude and phase so as to emulate the coupling channel responsible for interference in the Rx. However, when the self-interference is much stronger than the desired signal, the baseband cleaning approach suffers from saturation of the ADC of the Rx, whereas the power cancellation is inherently narrowband and may impact the Tx radiation efficiency [6]. Moreover, the total isolation of ∼60 dB obtained by combing active analogue and digital filtering is still not enough to provide full duplexing. Another approach, a focus of this work, can be found in [6], [11]-[14]. Therein, Tx-Rx isolation for a single-input-single- output (SISO) system is gained by reducing or canceling the coupling trans-impedance between the Tx and Rx ports, thus, unlike power cancellation or baseband cleaning, the approach selectively cancels the Tx power in the Rx direction before arriving at the Rx port. First, this is partially achieved by allocating separate antennas for the Tx and the Rx. Secondly, by equipping the Tx with redundant antenna elements or ports, the Tx signal can be properly weighted (either in baseband or the RF) and nulled in the Rx direction. Such a technique, herein referred to as corrective beamforming, reportedly provided ∼73 dB of wideband isolation in a SISO configuration which can be further enhanced by subsequent active digital or analogue filtering stages [6]. In this work, we take a step further to propose an an- tenna system with combined full-duplex and MIMO capa- bilities. Unlike [6], [11]-[14], the full-duplex MIMO system must be thoughtfully designed to achieve the required large isolation for full-duplexing (full-duplex isolation) within a MIMO setup. Therefore, with more than a single Tx and Rx communication ports for MIMO, the challenge here is to maintain significantly high isolation among all possible The 8th European Conference on Antennas and Propagation (EuCAP 2014) 978-88-907018-4-9/14/$31.00 ©2014 IEEE 2546